The present invention relates to the use of sheets for the production of molded articles according to the vacuum-forming or thermoforming method, made up of mixtures of polystyrene and diene-styrene block copolymers of the type S - B - S, wherein S represents the styrene structure and B represents the butadiene or isoprene structure.
The state of the art of impact-resistant types of polystyrene may be ascertained by reference to British Pat. No. 1,350,908 and W. German Published Patent Applications Nos. 1,569,311; 1,669,662; 1,694,879; 2,055,891; and 2,163,248, the disclosures of which are incorporated herein.
The state of the art of S - B - S block copolymers useful in the present invention may be ascertained by reference to the Kirk-Othmer "Encyclopedia of Chemical Technology", 2nd Ed., Supplement Volume (1971), -- 910-932, under the section "Styrene-Butadiene Solution Copolymers", particularly pp 916-932 wherein the preparation of block copolymers is disclosed, and U.S. Pat. Nos. 3,078,254; 3,251,905; and 3,265,765, the disclosures of which are incorporated herein.
The state of the art of thermoforming may be ascertained by reference to Kirk-Othmer ibid, Vol. 14 (1967), p. 305, and the article of Hanna & Lomax in Modern Plastics, Edition 36, Vol. 6, p. 111 (1959). The state of the art of processing of plastics may be ascertained by reference to Kirk-Othmer ibid, Vol. 15 (1968), pp. 802-807.
It is conventional to render polystyrene less susceptible to impact and shock stresses by the addition of elastomeric materials. Such so-called impact resistant polystyrene types are manufactured, as is known, by intermixing styrene homo- or co- or graft polymers with rubber-elastic compounds. However, it is also known to produce impact-resistant types of polystyrene by graft polymerization. It is furthermore basically conventional to utilize so-called butadiene-styrene segment or block copolymers as the elastomeric component which imparts impact resistance. In this connection, one differentiates between the so-called S -B and S -B -S types, wherein S represents the styrene structure and B represents the diene structure. The proportion of plasticizing butadiene or isoprene in these block copolymers generally varies between 50 and 85, preferably between 55 and 80 percent by weight.
The impact-resistant types of polystyrene obtained by the mixing of polystyrene and block copolymers have not as yet been satisfactory, however, in the manufacture of molded articles with respect to their impact strength. Thus, with the proportion of 8 percent by weight of block copolymer the notched impact strength is merely 3.9 cm .sup.. kg/cm, as disclosed in W. German Published Patent Application No. 1,569,311 at Table III.
Consequently, to improve this characteristic, the block copolymers have been mixed with other impact-resistant polystyrene types, such as disclosed in W. German Published Patent Applications Nos. 1,694,879 and 1,669,662. Mixtures have also been produced of impact-resistant polystyrene, homopolystyrene, and block polymers, as disclosed in British Pat. No. 1,350,908. Also, to improve the situation, attempts have been made to incorporate additionally a lubricant, as disclosed in W. German Published Applications Nos. 2,055,891 and 2,163,248. As can be seen from the state of the art, the molding compositions, in order to attain comparable values for the impact strength with customary impact resistant types of polystyrene, require additions of 20 and more percent by weight of block copolymers to the styrene homopolymer as compared to a rubber proportion of about 8 percent by weight with comparable values.